The thermal stability of ultrathin chromium films on Pt(111) has been studied using low-energy He+ ion scattering spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. At room temperature, Cr grows on Pt(111) with a modified Stranski-Krastanov growth mode (almost complete wetting of the first two layers with subsequent three-dimensional island growth). Upon annealing, chromium diffuses into the Pt lattice. This causes a smoothening of the surface features. A flat Pt(111) surface, devoid of Cr, is regained after prolonged annealing above 770 K. Ar lower temperatures, metastable Cr-Pt surface alloys are formed. The apparent composition and stability of these surface alloys are dependent upon the amount of Cr deposited initially. Both the Cr adlayer and Pt surface atoms were found to be perturbed at the interface. Experimental and theoretical results indicate that a redistribution of charge between Pt and Cr occurs upon alloy formation with the greatest perturbation experienced by the Pt atoms.